The present invention relates to a radio communication method and apparatus for multiplex transmission of plural signals in the same frequency band by use of a transmitting device composed of plural transmitters and a receiving device composed of plural receivers.
The realization of high-speed transmission as for video delivery in future mobile radio communications calls for a radio communication scheme of high frequency utilization efficiency. The frequency utilization efficiency is commonly defined by the ratio of traffic volume and the spectrum space used. What is meant by the “spectrum space used” is the product of the frequency bandwidth used, the dimension of the physical space occupied and the time consumed. To raise the frequency utilization efficiency, there are proposed mainly three schemes: (1) To increase the amount of information to be transmitted per unit frequency; (2) To reduce the cell radius to increase the number of reusable frequencies; and (3) To increase the number of usable frequencies per band by interleave channel assignment.
For example, in a digital modulation system, the amount of information transmitted per unit frequency can be increased by making transmission signals multilevel. An example of this scheme is an application of 16 QAM (Quadrature Amplitude Modulation) to 256 QAM in fixed microwave transmission. An example of reduction of the frequency bandwidth is an interleave channel assignment in mobile communications. An example of reduction of the physical space is the implementation of microcells in PHS (Personal Handyphone System). An example of reduction of the time used is the implementation of a half-rate speech coding based on full-rate speech coding in PDC (Personal Digital Cellular).
Various radio transmission techniques as described above are combined to enhance the frequency utilization efficiency. For example, PDC multiplexes three or six speech channels in a 3-sector 1-carrier frequency system by the combined use of such techniques as frequency assignment by the sector cell system, high-efficiency speech coding, a time division multiple access system and so forth.
The frequency utilization efficiency can be increased simply by closely spacing the frequency bands for use plural transmitters, but as the bands overlap more and more, signal separation becomes harder, resulting in increased interference and hence degrading the speech quality.